Electronic music keying circuit with diode and capacitor for reducing leakage current

ABSTRACT

An improved transistor keying circuit for use in electronic musical instruments to help minimize leakage of signals therethrough even when the circuit is in an off condition as is often caused by interelectrode transistor capacitances. The improved circuit utilizes a diode connected in series with a load resistor to a transistor collector or emitter electrode in a direction that permits collector-emitter current to flow normally in the forward direction thus permitting normal circuit operation when in an &#39;&#39;&#39;&#39;on&#39;&#39;&#39;&#39; condition. A capacitor is effectively shunt connected across the diode and load resistor with the output being taken from the junction of the load resistor and diode. Since the capacitor has a large capacitance as compared with the base-collector or base-emitter capacitance, leakage signals in the &#39;&#39;&#39;&#39;off&#39;&#39;&#39;&#39; state are effectively shunted to ground to greatly reduce the level of any leakage to the output terminal.

United States Patent [111 3,602,626

[72] Inventor Kazuyuki Aramaki 3,247,306 4/ 1966 Milho 84/1 13 tflmI Japan 3,383,453 5/1968 Sharp 84/1 .26 [21] Appl. No. 860,488 3,422,208 1 1969 Barry..... 84/ l .01 [22] Filed Sept. 24, 1969 3,465,088 9/ 1969 Kohls 84/ 1 .26 ntcd Aug. 31, 1971 3,535,430 10/1970 Maynard 84/1. 13 X [73] Assignee Nippon Gakki Seizo Kabushiki Kalsha 3,5 35,972 10/1970 Teranishi 84/1.26

glepresentative: Genichi Kowakami) FOREIGN PATENTS I amamatsu-shi, Japan priomy p 1968 1,164,422 9/1969 Great Britain 84/1 .26 [33] Japan Primary Examiner-Milton O. Hirshfield [31 43/84108 Assistant ExaminerStanley J. Witkowski [54] ELECTRONIC MUSIC KEYING CIRCUIT WITH DIODE AND CAPACITOR FOR REDUCING Attorney-Cushman, Darby and Cushman ABSTRACT: An improved transistor keying circuit for use in electronic musical instruments to help minimize leakage of signals therethrough even when the circuit is in an off condition as is often caused by interelectrode transistor capacitances. The improved circuit utilizes a diode connected in series with a load resistor to a transistor collector or emitter electrode in a direction that permits collector-emitter current to flow normally in the forward direction thus permitting normal circuit operation when in an on" condition. A capacitor is effectively shunt connected across the diode and load resistor with the output being taken from the junction of the load resistor and diode. Since the capacitor has a large capacitance as compared with the base-collector or baseemitter capacitance, leakage signals in the off state are effectively shunted to ground to greatly reduce the level of any leakage to the output terminal.

1 PATENTED'AUBBI l97| sum 1 or g Ccb C+Ccty out INVENTOR ATTORNEYS SHEETZUFZ TRn I INVENTOR BY T 7 MW v ATTORNEYS ELECTRONIC MUSIC KEYING CIRCUIT WITII DIODE. AND CAPACITOR FOR REDUCING LEAKAGE CURRENT The present'inventionis concerned with a keying circuit device for use in electronic musical instruments, and more particularly, it relates to an improved keying circuit which substantially reduces the signal leakage caused by the stray capacitance present between the collector and the base of a switching transistor of a keying circuit.

In electronic musical instruments, it has been the general practice to use, as the tone generators, square wave oscillators, since they contain a large amount of higher harmonic V waves suitable for forming various tone colors. The square wave signals which are oscillated by these oscillators are supplied to either a filter, an amplifier or a musical effect impressing circuit of various types through keying circuits as they one shown in FIG. 1. In the keying circuit shown in FIG. 1, the equivalent circuit of the signal passage when the key switch K is in its off" state is as shown in FIG. 2. It is noticed fromthe drawings that the capacitance Ccb which is the intercollector-base capacitance of the transistor is connected in series between the input and the output side. This capacitance is normally negligibly small. However, in view of the fact that the input signals of the keying circuit are square waves which are rich in higher harmonic components, these components tend to appear, as the leakage signals passing through the capacitance Ccb, on the output side. Accordingly, even in case the key switch is in its off state, the electronic musical instrument equipped with such a conventional keying circuit has the drawback that said higher harmonic components come out in the form of background noise.

' SUMMARY OF THE INVENTION It is, therefore, the object of the present invention to eliminate the foregoing drawback encountered in the keying circuits of conventional electronic musical instruments and to provide a keying circuit device which can substantially reduce the signal leakage caused by the capacitance between the collector and the base of a transistor.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of a typical conventional keying circuit device which has been used in electronic musical instruments;

FIG. 2 is an equivalent circuit diagram of the essential portion of the signal passage in FIG. 1 when the key switch is in its off" state;

FIG. 3 is an embodiment of the keying circuit device of the present invention;

FIGS. 4 and 5 are equivalent circuit diagrams when the key switch in FIG. 3 is in its ofi' state;

FIG. 6 is a brief diagram showing another embodiment in which the present invention is applied to a plurality of keying circuits; and

FIG. 7 and 8 are circuit diagrams showing still another and yet another embodiments of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will hereunder be described in further detail in connection with preferred embodiments by referringto the accompanying drawings.

In the drawings, FIG; 3 shows an embodiment of the keying circuit device of the present invention. This keying circuit device is of the arrangement comprising a diode D connected in series between the collector of a switching transistor TR and a load resistor Rc in such a way that said diode permits the collector current to flow in its forward direction (from the resistor Re to the collector, in this case), and a capacitor C connected between the collector and the earth E and having a relatively large capacitance as compared with the inter-collector-base capacitance Ccb.

FIGS. 4 and 5 show the-equivalent circuits for an instance where the key switch K'is in its off state in the circuit illustrated in FIG. 3. In FIG. 4', the symbol r represents the working resistance of the 'diode. Cd represents the junction capacitance of the diode. Since, in these circuits, the signal voltage which appears at point A in FIG. 4 represents a partial voltage resulting from the division of the input signal voltage Vin by the total of the capacitance C and the inter collectorbase capacitance Ccb, the signal voltage at point A will be: (Ccb/C +Ccb) Vin.

Also, the working resistance r resulting from the forward bias current of the diode D (or, in other words, resulting from the collector current of the transistor TR) will have a value which is considerably large when the key switch is in its "off" state, since the collector current of the transistor TR when the key switch is in its off state is much smaller than the current when said key switch is in its on" state. Besides, the working resistance in this instance will take a value which is relatively large as compared with the reactance value of the junction capacitance of the diode against the higherharmonic components of the square wave input signal, and therefore, said working resistance r may be ignored. In addition, the junction capacitance Cd of the diode is normally substantially equal to the inter-collector-base capacitance Ccb of the transistor. Accordingly, the equivalent circuit shown in FIG. 4 will become as that shown in FIG. 5. Thus, the keying circuit shown in FIG. 3 may, after all, be regarded as beingequal to the equivalent circuit shown in FIG. 5 in which the input signal voltage takes a value of VinXCcb/C+Ccb when the key switch K is in its off state.

More specifically, the keying circuit device of the present invention is arranged so as to cause also the diode to effect a switching action (meaning an increase in the impedance when the key switch is in its off state) by utilizing the difference in the magnitude of the working resistance r resulting from the changes in the forward bias current of the diode '(or, in other words, the changes in the collector current of the transistor) depending on the on-off state of the key switch K, and that, along with this, the leakage signal is effectively decreased to Ccb/C+Ccb time, while the key switch K is in its off position (compare the equivalent circuits in FIGS. 2 and 5), by the -.si di .aslie ist bath h ,sanes taass aqth inter-collectorbase capacitance Ccb, whereby reducing the leakage of tone signals while the key switch is in its off state.

FIG. 6 shows another embodiment of the present invention in which the collectors (output terminals) of the transistors of a plurality of keying circuits are connected in common relative to each other so that the leakage signal is reduced in common also by a set of diode and capacitor. In this embodiment, the working resistance-when the key switch is in its on" state, or, in other words, when there flows a large amount of forward current of the diode-is only ten ohms which is sufficiently small as compared with the output resistance of the collectors of the transistors which amounts to several hundred kiloohms. Therefore, even if the collectors are connected in common, there will arise no problem in actual operation. By arranging in this way, it becomes possible to substantially reduce, by a set of diode and capacitor, the noise which is produced from a plurality of keying circuits, and accordingly, this arrangement is advantageous in terms of cost.

FIG. 7 shows a still another embodiment of the circuit of the present invention in which the square input signal is applied to the emitter of the transistor TR so that the opening and closing of the key switch is adapted to be performed by the base circuit.

FIG. 8 is a yet another embodiment of the present invention in which the transistor is arranged to serve as an emitter follower.

In both of these two embodiments, the circuits operate in the similar way as does the keying circuit shown in FIG. 3.

I claim:

1. An improved transistor switching circuit for use in electrical musical instruments and which minimizes undesired leakage of signals from an input terminal to an output terminal in an off state due to internal interelectrode capacitances of the switching transistor, said improved circuit comprising:

ofsaid diode and load resistor. 3. A keying circuit according to claim 2 wherein: said transistor comprises an emitter-grounded amplifier,

at least one switching transistor'having an input electrode said input electrode is a base electrode of said transistor,

and an output electrode with interelectrode capacitance 5 said output electrode is a collector electrode of said therebetween, transistor, and

an input terminal for receiving input signals, said key switch is connected in series in the emitter path of I an output terminal for providing an output signal corsaid transistor. responding to said input signal when said circuit is 4. A keying circuit according to claim 2 wherein: switched on, '10 said transistor. constitutes a base-grounded amplifier,

a diode connected to said output electrode with a forward said input electrode "is an emitter electrode of said diode current direction corresponding to forward collectransistor, tor-emitter current in said transistor to insure normal said output electrode is a collector electrode of said switching operation when the circuit is switched on for transistor, and passing signals from said input terminal to the output tersaid key switch is connected in series in the base path of said minal but which offers a relatively large impedance when transistor. the circuit is switched off, r v 5. A keying circuit according to claim 2 wherein:

a load resistor series connected withsaid diode at a junction said transistor constitutes a collector-grounded amplifier, point, said input electrode is a base electrode of said transistor,

said output terminal being connected to said junction point, aid output electrode is n emitter electrode f Said and transistor, and j a capacitor having a large capacitance a compared t id said key switch is connected in series in the emitter path of interelectrode capacitance, said capacitor being effecsaid transistor. I tively shunt connected across said series connected diode Y f; electromc musical mm i and resistor thereby effectively minimizing undesired sald F W P m leakage of signals to the output terminal in the off state of a plurahty ofmput f s said circuit by shunting such signals away from the output a output F 1 terminal r 1 a plurality of transistors and a common load resistor com- 2. A keying circuit for use in electronic musical instruments, Pnsmg common load P r Said circuit comprising; one electrode of each of said transistors effectlvely'bemg an an input terminal; output electrode, a I

an output terminal; a second electrode of each of said transistors effectively a transistor and a load resistor constituting an amplifier cirbemgfm mput eletmdg cuit, a piurality of key sw tches each being connected in each of one electrode of said transistor effectively being an input Said pluralityPfamPlifier circuits l Posltio," where electrode and another electrode of said transistor effec- P each key f" render assoc'ated tively being an ouput electrode, amplifier operative and nonoperative,

a key switch connected in said amplifier circuit at a position a dfode connefted m genes 831d where closing and opening of said key switch renders said d Sam output eleclrqdes m Such a dlrecmn as F amplifier operative and nonoperative, I v 4 permit normal forward transistor currents to also flow in a diode connected in series between said load resistor and the fqrward dlrfctmn ofsad diode; and i said output electrode in such a direction as to permit the a common connectefj bgtweewsald output g normal forward transistor current to also flow in the fortrodes and a constant Ptent lal n a subsmnuauy ward direction of said didde; and larger capacitance than the internal capacitance between.

a capacitor connected between said output electrode anda f electrode and sad Output electrode of of constant potential terminal, said capacitor having a sub- F tranS'stt S; p stantially large capacitance than the internal capacitance input termufals bemg connected to P elec between said input electrode and said output electrode of nodes and a common outpu teljmmal Said transistor; nected to the uncture between said diode and said load said input terminal being connected to said input electrode res'stor" and said output terminal being connected to the juncture 

1. An improved transistor switching circuit for use in electrical musical instruments and which minimizes undesired leakage of signals from an input terminal to an output terminal in an off state due to internal interelectrode capacitances of the switching transistor, said improved circuit comprising: at least one switching transistor having an input electrode and an output electrode with interelectrode capacitance therebetween, an input terminal for receiving input signals, an output terminal for providing an output signal corresponding to said input signal when said circuit is switched on, a diode connected to said output electrode with a forward diode current direction corresponding to forward collector-emitter current in said transistor to insure normal switching operation when the circuit is switched on for passing signals from said input terminal to the output terminal but which offers a relatively large impedance when the circuit is switched off, a load resistor series connected with said diode at a junction point, said output terminal being connected to said junction point, and a capacitor having a large capacitance as compared to said interelectrode capacitance, said capacitor being effectively shunt connected across said series connected diode and resistor thereby effectively minimizing undesired leakage of signals to the output terminal in the off state of said circuit by shunting such signals away from the output terminal.
 2. A keying circuit for use in electronic musical instruments, said circuit comprising: an input terminal; an output terminal; a transistor and a load resistor constituting an amplifier circuit, one electrode of said transistor effectively being an input electrode and another electrode of said transistor effectively being an output electrode, a key switch connected in said amplifier circuit at a position where closing and opening of said key switch renders said amplifier operative and nonoperative, a diode connecTed in series between said load resistor and said output electrode in such a direction as to permit the normal forward transistor current to also flow in the forward direction of said diode; and a capacitor connected between said output electrode and a constant potential terminal, said capacitor having a substantially large capacitance than the internal capacitance between said input electrode and said output electrode of said transistor; said input terminal being connected to said input electrode and said output terminal being connected to the juncture of said diode and load resistor.
 3. A keying circuit according to claim 2 wherein: said transistor comprises an emitter-grounded amplifier, said input electrode is a base electrode of said transistor, said output electrode is a collector electrode of said transistor, and said key switch is connected in series in the emitter path of said transistor.
 4. A keying circuit according to claim 2 wherein: said transistor constitutes a base-grounded amplifier, said input electrode is an emitter electrode of said transistor, said output electrode is a collector electrode of said transistor, and said key switch is connected in series in the base path of said transistor.
 5. A keying circuit according to claim 2 wherein: said transistor constitutes a collector-grounded amplifier, said input electrode is a base electrode of said transistor, said output electrode is an emitter electrode of said transistor, and said key switch is connected in series in the emitter path of said transistor.
 6. A keying circuit for use in electronic musical instruments, said circuit comprising: a plurality of input terminals; a common output terminal; a plurality of transistors and a common load resistor comprising common load amplifiers, one electrode of each of said transistors effectively being an output electrode, a second electrode of each of said transistors effectively being an input electrode, a plurality of key switches each being connected in each of said plurality of amplifier circuits at a position where closing and opening of said each key will render its associated amplifier operative and nonoperative, a diode connected in series between said common load resistor and said output electrodes in such a direction as to permit normal forward transistor currents to also flow in the forward direction of said diode; and a common capacitor connected between said output electrodes and a constant potential and having a substantially larger capacitance than the internal capacitance between said input electrode and said output electrode of one of said transistors; said input terminals being connected to said input electrodes, and said common output terminal being connected to the juncture between said diode and said load resistor. 